This invention relates to electric motor control systems, and more particularly, to systems for controlling AC induction motors over a broad range of speeds and conditions.
In applications where variable speed electric motors are required, it has generally been the practice to utilize DC motors because of the ability to accurately control such motors over a broad range of speeds and conditions. In DC motors the winding current controls the motor torque and can be directly measured to achieve accurate control and the desired operation.
In an AC induction motor the torque is a function of induced current in the rotor which in turn is a function of slip, i.e., the difference in speed between the rotor and the rotating magnetic field produced by the stator. The speed of the rotating magnetic field is determined by the frequency of the winding energizing current. However, due to the slip, the rotor speed will differ therefrom by a variable amount related to the torque demands on the motor. As a result, it is difficult to accurately control rotor speed of an AC induction motor under variable torque conditions. Thus, even though AC induction motors are considerably less expensive than DC motors, they are generally not used where accurate speed control is required.
An objective of this invention is to provide a motor control system capable of accurately controlling the speed of an AC induction motor over a broad range of speeds and torque conditions.